Numerous methods have been developed over the years to produce isoprene. One method described in U.S. Pat. No. 4,524,233, herein incorporated by reference, discloses the dehydration reaction of 2-methylbutanal (2MBA) over a dehydration catalyst such as boron phosphate to yield isoprene. As disclosed in U.S. Pat. No.4,524,233, a major by-product of the 2MBA dehydration is methylisopropylketone (MIPK). For the economical operation of a process as described in this U.S. patent, it will be required to recycle the 2MBA that is not converted to MIPK or isoprene, back to the dehydration reactor.
The close boiling points of 2MBA (90.degree.-92.degree. C.) and MIPK (92.degree.-94.degree. C.) make separation by distillation difficult. A separation of two compounds having close boiling points is known to those skilled in this art to present extreme difficulties.
The inventor herein has unexpectedly found that a mixture of aldehyde such as 2MBA and a ketone such as MIPK can be separated after acid-catalyzed cyclotrimerization of the aldehyde. The ketone and unreacted aldehyde are distilled away from the cyclotrimerized aldehyde easily and efficiently and thereafter, the cyclotrimerized aldehyde can be catalytically reconverted to the starting aldehyde in high purity. None of the prior art suggests or discloses this unique approach to the separation of close boiling aldehydes and ketones.
U.S. Pat. No. 4,163,696 is concerned with a distillation process for the recovery of methylisobutyl ketone. This patent discloses an azeotropic distillation process for separating toluene from methylisobutyl ketone in a spent liquor mixture. A toluene azeotrope former, preferably methanol, is added to the liquor in an amount sufficient to form an azeotrope with all the toluene present in the mixture.
It is known that when catalyzed by acids, low molecular weight aldehydes add to each other to form trimers. See, for example, Bevington, J C, Quart. Rev. (London) 6, 141 (1952). Although these cyclic acetals are stable to bases, they can be converted back to monomeric aldehydes by acids with heat. The following is a structural representation of the acid-catalyzed cyclotrimerization of an aldehyde: ##STR1## wherein R can be a hydrocarbon radical of 1 to 16 carbon atoms.
These acid-catalyzed cyclotrimerizations of the aldehyde do not affect the ketone or other nonaldehyde components. The trimers have substantially higher boiling points than their monomers.
A portion of the instant invention resides in the application of this unique property of aldehydes to the specific problem of separating close boiling mixtures of an aldehyde and a ketone. The unique process of this invention provides a means for separating these close boiling components. The aldehyde is selectively trimerized to a high boiling trimer at low temperatures by an insoluble solid acid catalyst. After removing the catalyst by filtration or other means, the aldehyde trimer is separated from the ketone and unreacted aldehyde by stripping or distilling the low boiling point compounds from the filtrate. The desired aldehyde is then recovered by heating its trimer with a solid acid catalyst to convert it back to the monomeric aldehyde species. The thus obtained aldehyde is of high purity.